The present invention relates to an acrylic foam pressure sensitive adhesive, particularly, an acrylic foam pressure sensitive adhesive, having a low density and having cells finely and uniformly dispersed therein and, more particularly, an acrylic foam pressure sensitive adhesive having a cell average diameter of no more than 300 xcexcm and its standard deviation of no more than 115 xcexcm, and a method of production thereof.
Acrylic foam pressure sensitive adhesives have been produced by chemical foaming methods in which a foaming agent is foamed in an acrylic solution prior to curing followed by its curing, or by mechanical foaming methods in which an inert gas such as nitrogen gas is dispersed and mixed into an acrylic solution prior to curing followed by its curing. Mechanical foaming methods can be superior in that the solution prior to coating can have high stability, the cell size can be easily controlled, the foam density can be easily controlled, cell uniformity can be comparatively high, closed cells can be formed having good sealing, the foam thickness can be easily controlled, and a foam can be obtained that is free of residues.
Foam pressure sensitive adhesives typically have superior cushioning, sealing and adhesion, etc. the lower the density and the greater the number of uniform cells. Consequently, in mechanical foaming methods, it is important to improve the performance of the process in which inert gas is finely and uniformly dispersed and mixed into the solution prior to curing. Methods currently used employ an apparatus equipped with a stator having a large number of fine teeth on a disk having a through hole in its center, and a rotor positioned in opposition to the stator having a large number of fine teeth similarly having a large number of fine teeth on a disk. By passing the acrylic solution prior to curing and cell forming gas (e.g., nitrogen gas) from the through hole into the gaps between the teeth on the stator and the teeth on the rotor, and rotating the rotor at high speed, the cell forming gas is finely dispersed and mixed into the acrylic solution. According to the method of the prior art which uses this apparatus (to be referred to as the xe2x80x9capparatus of the prior artxe2x80x9d), the average diameter of the cells of the resulting foam typically exceed 300 xcexcm, and its standard deviation exceeds 115 xcexcm. In addition, the foam density is no less than about 0.60 (g/cc) even in the lowest cases.
An object of the present invention is to provide an acrylic foam pressure sensitive adhesive having a low density and in which the cells are finely and uniformly dispersed unlike that obtained with the method of the prior art.
In one aspect of the present invention, an acrylic foam pressure sensitive adhesive is provided having a cell average diameter of no more than 300 xcexcm and its standard deviation of no more than 115 xcexcm.
In another aspect of the present invention, a method for producing an acrylic foam pressure sensitive adhesive is provided that comprises introducing an acrylic foam precursor composition and a cell forming gas into an agitation and mixing apparatus equipped with a casing provided with a flow path through which a fluid flows, and agitation blades disposed within said casing being capable of vibrating in the axial direction of said casing, forming cells within the acrylic foam precursor composition by agitating and mixing the acrylic foam precursor composition and the cell forming gas in the agitation and mixing apparatus, coating the acrylic foam precursor composition containing the cells onto a substrate, and curing the coated acrylic foam precursor composition.
An acrylic foam pressure sensitive adhesive is obtained by the above specific agitation and mixing apparatus that uses the vibration principle (to be referred to as a vibrating agitation and mixing apparatus). This foam pressure sensitive adhesive has higher adhesive strength and cushioning as compared with foam pressure sensitive adhesives of the prior art, and consequently has superior sealing and sticking capabilities.